Steam generators of pressurized water nuclear reactors include tubes which are bent into a U having two straight branches crimped at their ends into a tube plate. The straight branches of the tube are held by tube support plates spaced along the length of the straight branches and through which the straight branches pass. The tubes of the bundle are thus held in a regular arrangement in which the straight branches are all parallel to each other and the tubes arranged in plane rows which are parallel to each other, in each of which the tubes whose bent parts have radii of curvature decreasing from the outside towards the inside of the row are juxtaposed, the ends of the tubes of one row being engaged in a rectilinear line of openings passing through the tube plate.
The tubes in one row and the adjacent rows are separated by narrow free spaces allowing the feedwater of the steam generator to pass inside the bundle, in contact with the outside exchange surface of the tubes of the bundle.
Furthermore, the tubes situated at the central part of each of the rows and which include the bends of smallest radius of curvature are aligned, so as to delimit between them a free space whose lower part, above the tube plate of the steam generator, constitutes the tube lane of the steam generator.
The curved parts of the tubes of each of the rows of the bundle have different radii of curvature and are juxtaposed so as to constitute a structure of substantially hemispherical shape, termed the tube bend region, at the upper part of the bundle of the steam generator.
During operation of the steam generator, water under pressure and at high temperature flows through the tubes of the bundle and feedwater is brought into contact with the external exchange surface of the tubes along which it moves in the vertical direction while heating up and then vaporizing, to emerge in the form of steam at the upper part of the steam generator.
The flow of the fluids in contact with the tubes may cause vibrations which can lead to damage to the tubes if they are not held efficiently.
The straight part of the tubes which is engaged in the tube support plates is held efficiently by the supports made in the form of rigid plates. The curved parts of the tubes of the bundle constituting the tube bend region should also be held, and anti-vibration bars are generally used for this which are interposed between the adjacent rows of tubes of the bundle and arranged in substantially radial directions of the tube bend region. These anti-vibration bars, as described for example in U.S. Pat. No. 3,007,679, may be folded or assembled so as to have the shape of a V whose two branches are directed in use along the radial directions of which the tube bend region between the rows of tubes of each of the pairs of adjacent rows.
The ends of the branches of the anti-vibration bars opposite their common ends are generally projecting with respect to the tubes constituting the external layer of the tube bend region and connected together by curved bars resting on the external surface of the tube bend region on which the external end parts of the anti-vibration bars are welded.
Other ways of fastening the external ends of the anti-vibration bars have also been proposed more recently; these might employ removable fastening elements, such as screwed elements.
It has also proved necessary to provide means for fastening and connection of the internal part of the anti-vibration bars, in the vicinity of their folded or articulated part, onto a structural element of the steam generator.
Applicants' US-A-5,269,371, means were provided for fastening the anti-vibration bars to prevent their ejection out of the bundle in the generator in use, which means comprise an elongate structure fixed onto the tube support plate of the steam generator nearest the curved parts of the tubes constituting the tube bend region. The elongate structure, generally constructed in the form of a rail, is fixed onto the upper face of the tube support plate, in a direction perpendicular to the rows of tubes, inside the central free space constituting the tube lane of the steam generator. The elongate fastening structure includes, at each of the spaces between two couples of adjacent tube rows, at least one notch in which the internal part of the anti-vibration bar can be engaged.
Between two adjacent rows of the bundle, three V-shaped anti-vibration bars are generally arranged, including in total six branches which should be placed in radial directions of the tube bend region with some degree of angular distribution.
It is therefore necessary to place, at spaces between two adjacent rows of tubes of the bundle and possibly to fasten into notches of the elongate element, three double anti-vibration bars with angular spacings between their different branches.
The design and the structure of the elongate fastening element are relatively complex because steam generators include a large number of rows of tubes, for example of the order of 130.
It is therefore necessary to provide a very large number of notches machined in the elongate fastening element.
Applicants' FR-A-2,603,364 a method for fitting tubes of a steam generator in successive rows was provided, in which the anti-vibration bars are set bearing on the curved parts of the tubes of the last row which has been fitted, with controlled bearing forces substantially corresponding to the bearing force of the rows intended to rest on the row during mounting. It is checked that the anti-vibration bars subjected to the bearing forces actually are in contact with all the tubes of the row, and the defective tubes on which the anti-vibration bars cannot bear are possibly eliminated. Defect-free mounting of the steam generator, and in particular of the anti-vibration bars which bear perfectly on the curved parts of the tubes constituting the successive rows of the bundle is thus achieved.
It is clear that the mounting of the bundle of the steam generator is facilitated and improved when an internal element for fastening the anti-vibration bars is used, such as an elongate element including notches, solidly attached to the upper tube support plate of the steam generator. However, because each of the anti-vibration bars must be fixed onto the elongate anti-fly-off holding element separately, the operations of fitting the anti-vibration bars remain relatively lengthy, because of the large number of rows and anti-vibration bars in the bundle of the steam generator.